1. Field of the Invention
The present invention relates to skew correction devices, image forming systems, and skew correction methods. In particular, the present invention relates to a skew correction device that performs skew correction on a booklet produced by folding sheets of a recording medium such as ordinary paper, recording paper, or transfer paper (hereinafter, abbreviated to “sheets”), an image forming system including the skew correction device, and a skew correction method to be performed by the skew correction device.
2. Description of the Related Art
There are widely known sheet processing apparatuses arranged downstream of an image forming apparatus body to perform postprocessing, such as stapling, on recording sheets or the like output from the image forming apparatus. Such sheet processing apparatuses have come to incorporate multiple functions recently, and sheet processing apparatuses capable of not only side stitching as in conventional apparatus but also saddle stitching and making booklets have become common. Some type of sheet processing apparatuses, which perform saddle stitching and booklet making, further cut an edge(s) of a stapled booklet using a cutting apparatus to increase quality of outputs.
In such a cutting apparatus, a booklet to be cut is typically conveyed by a conveying unit such as a belt and positioned by being abutted against a positioning stopper set to adapt to a size of the booklet, a cut amount, and/or the like. Thereafter, a cutting unit cuts an edge of the booklet that is pressed and fixed by a pressing unit. Thereby, an edge of the stapled booklet is evenly trimmed.
However, in the conventional cutting apparatus, the conveying unit such as a belt rotates in a state where the booklet is in contact with the positioning stopper. This can cause a surface sheet of the booklet to be swollen out. When such a booklet is cut as-is, the side of the cut booklet becomes uneven. Furthermore, when the pressing section presses the booklet in a manner to flatten the swelling of the booklet, a front side or a back side of the booklet can be warped while the booklet is pressed by the pressing unit, and possibly further resulting in that the booklet is moved forward or backward and pressed in a misaligned state. Accordingly, cutting the booklet in such a pressed state can undesirably make the edge of the cut booklet uneven.
Examples of known techniques for preventing such an uneven edge include a technique disclosed in Japanese Patent No. 3472772. According to this technique, an apparatus conveys a booklet using an endless belt such that a spine of the booklet is a leading end, includes, at a downstream part, a stopper plate on which the spine of the booklet is to abut and an aligning reciprocating piece for back-jogging, and aligns the booklet by lightly pressing a fore edge of the booklet toward a leading end of the booklet. It is argued in Japanese Patent No. 3472772 that the apparatus is highly productive because the apparatus configured as described above can process and convey booklets downward one after another.
According to the technique disclosed in Japanese Patent No. 3472772, the apparatus includes the aligning reciprocating piece for back-jogging that aligns and positions a booklet by jogging a fore edge of the booklet and is configured to align the booklet by lightly pressing the fore edge of the booklet toward the leading end of the booklet. However, a saddle-stitched booklet can generally have an error in length, which may be caused from an error in length of not-yet-center-folded sheets, a center-folding error resulting from deviation in folding position, or an error that a booklet becomes practically shorter when the booklet is not folded completely flat and swollen out. When a booklet has a large error in length, it is difficult to lightly press a fore edge of the booklet with the aligning reciprocating piece for back-jogging-alignment only by position control.
Under the circumstance, a spring or the like can be provided on a jogger to perform alignment or skew correction with a constant pressure when sheets to be aligned have wide variation in sheet size. It is easy to lightly press a fore edge of a booklet using this approach.
However, an appropriate pressure to be applied to a fore edge for alignment varies depending on paper type, paper thickness, number of sheets to be stapled, and a folding height. For instance, when a pressing spring force is insufficient, the spine of the booklet fails to abut on a stopper plate, resulting in misalignment. In contrast, when the pressing spring force is excessively large, the spring shows no resiliency after the booklet has abutted, causing buckling of the booklet leading to damage such as bent and/or scratch. Accordingly, booklets to which alignment or skew correction with certain pressure is applicable are limited. Meanwhile, “abutting” in this document means that something comes into contact with an object and this abutted (contact) condition is maintained. “Pressing” means generating a pressure by pressing. A “pressing force” is a pressure generated by pressing, or, in other word, a force exerted to press something. “At application of pressure” refers to time when an operation of pressing something is performed, or, in other words, when something is pressed.
There is a need to make it possible to perform skew correction on a booklet conveyed to a skew correction device reliably without causing a damage such as misalignment, crease, bent, and/or scratch